#include <iostream>
#include <cstring>
using namespace std;

#define MAX_N 20
long long dp[MAX_N + 5];

int main()
{
    int n;
    cin >> n;
    dp[1] = 1;
    for(int i = 2; i <= n; i++)
    {
        dp[i] = dp[i - 1] + i - 1;
    }
    // for(int i = 0; i <= n; i++)
    //      cout << dp[i] << " "; cout <<endl;
    cout << dp[n];
    return 0;
}